GCSE Chemistry: Rate, Equilibrium, and Reversible Reactions

Rate of reaction

Amount of product used or amount of product formed over time.

Rate of reaction formula (reactants)

Rate of reaction = amount of reactant used / Time

Rate of reaction formula (products)

Rate of reaction = amount of product formed / Time

Measurement of quantity

Quantity of reactant or product can be measured by mass in grams or by a volume in cm³.

Units of rate of reaction

Units may be given as g/s or cm³/s.

Moles in rate of reaction

Quantity of reactants can be in terms of moles, with units for rate of reaction in mol/s.

Graphical rate of reaction

To find the rate of reaction graphically, draw tangents to curves and use the slope of the tangent.

Factors affecting reaction rates

Concentration, pressure, surface area, temperature, catalysts.

Collision theory

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

Activation energy

The minimum amount of energy that particles must have to react.

Effect of concentration on reaction rate

Increasing concentration increases the frequency of collisions and the rate of reaction.

Effect of temperature on reaction rate

Increasing temperature increases the frequency of collisions and makes collisions more energetic.

Catalysts

Substances that speed up chemical reactions without being changed or used up during the reaction.

Enzymes

Act as catalysts in biological systems.

Catalysts in reaction equations

Catalysts are not included in the equation for a reaction.

Catalysts and activation energy

Catalysts decrease the activation energy, increasing the proportion of particles with energy to react.

Reversible reactions

Products can react to produce the original reactants.

Reversible reaction symbol

Use the symbol ⇌ instead of → to represent a reversible reaction.

Haber Process example

Hydrogen + nitrogen ⇌ ammonia.

Endothermic and exothermic reactions

If a reversible reaction is endothermic one way, it is exothermic in the opposite direction.

Equilibrium in closed systems

Equilibrium is reached when reactions occur at exactly the same rate in each direction.

Le Chatelier's principle

If a system at equilibrium experiences a change, it responds to counteract the change.

Effect of changing concentration on equilibrium

Changing concentration shifts equilibrium towards products or reactants until equilibrium is reached again.

Effect of temperature changes on equilibrium

Increasing temperature shifts equilibrium in the direction of the endothermic reaction.

Effect of pressure changes on equilibrium

An increase in pressure favors the reaction that produces the least number of molecules.

Pressure and yield of reaction

An increase in pressure decreases yield of reaction if it produces a larger volume of gas.